The present invention relates to high efficiency air filters, and more specifically to a filter core and frame assembly suitable for use in high temperature environments.
Air filters of the so-called HEPA type are required to operate with a minimum 99.97% efficiency in removing airborne particles. In a popular construction of such filters, a filter core is formed of a continuous sheet of filter media folded in zig-zag fashion to form a plurality of accordion pleats of uniform size with thin, corrugated, spacer sheets placed between the opposing walls of each pleat. The filter core is supported by a surrounding, box-like frame with which the media is sealingly engaged about its entire periphery, including both pleat ends and the entire length of both media edges. The integrity of the seal between filter core and frame is critical, of course, in preventing unfiltered air from bypassing the filter.
In some applications, air filters of this type are installed in locations where they are exposed to high temperatures, e.g., on the order of 1000.degree. F. Problems which have been encountered with typical filter assemblies employed under such conditions include separation of and consequent air leakage between sealers and frame members, cracking of sealers, and powdering or crumbling of adhesive. These problems may be due both to the high temperatures and to poor structural strength of the filter assembly, resulting from dissimilar rates of thermal expansion, poor adhesion and low strength.
An early approach to the problem of providing the required sealing between the filter core and frame which is capable of withstanding high temperatures is that disclosed in U.S. Pat. No. 2,884,091 of Baldwin. In this construction, a resilient mat or blanket of fine glass fibers is compressed between the filter core and a surrounding metal frame. However, after prolonged exposure to temperature approaching 1000.degree. F. the glass fibers begin to anneal, causing the mat to loose resiliency and eventually resulting in air leakage between the core and frame. Another filter construction intended for high temperature applications is that disclosed in U.S. Pat. No. 4,199,387 of Hladik, involving the application of a ceramic adhesive with a trowel to the zig-zag edges of the media and interposed spacer members.
Filter constructions and methods of assembly which include immersion of the zig-zag edges of the media in a liquid sealant which later hardens to form a seal between the media and frame are shown in U.S. Pat. No. 3,581,479 of Goulet, and 4,227,953 of Wasielewski and Hladik. In the former, a groove or channel is formed across the inner surfaces of the top and bottom frame members for insertion of an elongated nozzle through which a liquid adhesive is injected as the nozzle is withdrawn from the groove. The assembly method of the latter patent involves filling shallow pans with the liquid adhesive and immersing therein the zig-zag edges of the pleated media. The bottom members of the shallow pans become the ends of the filter casing, and side members are adhesively sealed to the end media pleats and attached to the end members of the casing.
The principal object of the present invention is to provide a HEPA air filter construction which retains its structural rigidity and filtering efficiency when exposed to high temperatures for relatively long time periods.
Another object is to provide a high efficiency air filter having improved means, effective at both normal and high temperatures, for maintaining an accordion pleated filter media in sealed engagement about its entire periphery with a surrounding frame.
A further object is to provide an air filter frame construction having improved features enhancing the ability of the frame to be placed and maintained in sealing engagement with a pleated filter core by means of a sealant initially in liquid form and hardenable to an air-impervious solid.
Other objects will in part be obvious and in part appear hereinafter.